It is often desirable to produce compressive stresses in spherical objects such as spherical bearing elements, sometimes referred to hereinafter as “balls.” This can be challenging when the balls are in a near-finish condition because known methods of imparting stress may damage the surface finish in a manner that requires reworking. Current methods for imparting compressive stress include tumbling and peening operations.
In a tumbling operation, a plurality of balls is placed in a horizontal or inclined drum, and the drum is rotated so that the balls are repeatedly lifted toward the top of the drum and dropped onto the bottom. The force of the ball impacting against the drum (or other balls in the drum) changes the characteristics of the portion of the ball that impacts against the drum or other balls. As the balls are repeatedly lifted and dropped, different surfaces of the balls absorb the impact forces until a somewhat uniform distribution of compressive stress has been imparted to the overall surface of each ball.
One disadvantage of tumbling operations is the amount of noise produced. On an industrial scale, the noise is so loud that these operations must be performed in special rooms or buildings that are soundproof, and workers cannot enter these buildings or rooms without hearing protection. It is also difficult to achieve a uniform distribution of stress/hardness across the entire spherical surface.
In a peening operation, balls are repeatedly accelerated and released so that they impact against a fixed surface or a media such as chilled cast iron shot is accelerated to impact the surface of the balls. This process is also very noisy and provides only limited control over the surface hardness and stress profile of the balls.
It would therefore be desirable to provide a method and apparatus for imparting compressive stress to surface portions of spherical balls in a manner that substantially avoids the foregoing problems.